In certain fields of application, large numbers of injections have to be made within a region of skin of a patient. One example of such field of application is cosmetic treatment where e.g. dermal fillers in the form of gels of hyaluronic acid are injected into the tissue of a patient in order to fill out undesirable wrinkles and similar. In the prior art, injection is typically done by using a syringe fitted with a hypodermic needle having a sharp, beveled tip. One challenge during such injection is to avoid bruising of the skin tissue or tissue trauma. This is especially relevant when visible skin regions such as the face, hands or décolletage of a patient is treated. Another drawback with the use of a traditional syringe in that type of treatment is that it can be time consuming considering the large number of injections sometimes required. In another prior art method, a number of incisions are made over a surface to be treated with a first instrument, e.g. a scalpel or a sharp hypodermic needle. Then, in a following stage, a blunt cannula is introduced through these openings and a liquid or gel composition is injected. That prior art method does however involve a number of drawbacks. It is inexpedient and inconvenient to have to use two different instruments during the work and if a large number of injections are necessary, that method is very time consuming. Further, the risk of cutting too deep in the first stage is rather high which increases the risk of post-treatment bruising and tissue trauma. Another non-negligible disadvantage with that method is the fact that it can be quite hard to find the incisions when the blunt cannula is to be inserted since the opening is rather small. Prior art document U.S. Pat. No. 5,098,389 discloses a needle assembly having a handle with a short introducer needle mounted to the handle, and a blunt cannula mounted within the handle and slidable within the handle. The needle assembly is held at the handle with one hand, and pushed against the skin. The introducer needle thereby cuts an incision. Then the cannula is inserted with the other hand. Such a two-hand operation is undesirable. Additionally, the needle assembly is not constructed for multiple injections. Prior art document U.S. Pat. No. 3,840,008 discloses a needle having a pointed hollow piercing member slidably mounted about a blunt cannula. The piercing member has a collapsible bar, which is finger operated. After having introduced the piercing member while holding the finger on the bar, the user removes the finger and inserts the cannula. During the last mentioned part of the operation the bar collapses. The needle is difficult to use and is not constructed for multiple injections. Another example is disclosed in GB 624269, where an injection needle has an outer sharp needle and an inner blunt cannula. The injection needle is a typical single injection device.